The moving labyrinth of walkways and machinery is currently 45 feet below the MacArthur Causeway on a 5 percent grade, headed down to an eventual 120 feet.

The giant cutting wheel on the front of the machine is about to encounter a layer of earth crews call "Strata 5," which they say is the hardest limestone they've encountered yet.

"Today, we're going under maintenance to make sure the cutter blades, the face shield and everything is in place to properly deal with that," said Christopher Hodgkins, vice president of the Miami Access Tunnel consortium.

The machine runs on electricity and hydraulics.

The 52,000 tons of hydraulic pressure that moves the boring machine forward also pushes back onto each concrete ring as its set into place.

"We're not just building frame," said Hodgkins. "We're putting in conduit, fire flow, pressurized water, pressurized air, and all of this is taking place as we're building the tunnel. We're trying to complete as much of the tunnel as we can as we go through the process."

Crews aim to complete 20 feet of tunnel a day.

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